Combined chlorine agent and production and use thereof

ABSTRACT

A combined chlorine agent having a low concentration of free chlorine and a high concentration of combined chlorine whereby combined chlorine concentration of water can be heightened when added to water systems at a low concentration of free chlorine, methods of efficient production and chlorine treatment at low free chlorine concentration are provided. The combined chlorine agent consists of an aqueous agent which comprises alkali consisting of alkali metal hydroxide, sulfamic acid, and oxidizing agent based on chlorine, wherein the composition ratio of the oxidizing agent based on chlorine to the sulfamic acid is in the range from 0.45 to 0.6 by Cl/N (mole ratio), the composition ratio of the oxidizing agent based on chlorine to alkali is in the range from 0.3 to 0.4 by Cl/alkali metal (mole ratio), and free chlorine concentration in the aqueous agent is 2% by weight or lower of total chlorine concentration.

TECHNICAL FIELD

The present invention relates to a combined chlorine agent used suitablyfor a slime control agent of a reverse osmosis membrane (in thefollowing sometimes called as “RO membrane”) and methods of productionand use thereof. More specifically, the present invention relates to acombined chlorine agent having a low concentration of free chlorine anda high concentration of combined chlorine, to a method of efficientproduction thereof and a method of chlorine treatment using the combinedchlorine agent.

TECHNICAL BACKGROUND

Since a RO membrane reveals a high rejection rate of solute, clearpermeate obtained by a RO membrane treatment has a superior waterquality and can be reused effectively for a variety of purposes. If,however, water to be treated contains contaminants such as suspendedsolids and organic substances which foul the RO membrane, there is aproblem wherein the RO membrane is contaminated by these contaminantscausing declines of flux and rejection rate as the treatment lasts.

In water treatment using a RO membrane, a method continuing a stableoperation by averting obstruction such as decline of flux and increaseof operational pressure in a RO membrane apparatus is performed byadopting pretreatment (including coagulation, solid-liquid separationand active carbon treatment) to clarify feed water to the RO membrane,wherein, in order to prevent fouling of the RO membrane and to obtain ahigh treatment efficiency, the feed water to the RO membrane apparatusis evaluated by a Fouling Index (FI) ruled in JIS K3802, or Silt DensityIndex (SDI) ruled in ASTM D4189, and the pretreatment is carried outwherein FI or SDI falls in a predetermined value or lower, for instance,FI or SDI becomes 3-4 or lower.

In cooling water circulation system, a part of water evaporates whencooling water used for cooling a heat source is cooled in a coolingtower, whereby contaminants such as suspended solids and organicsubstances are condensed. Further, to the cooling water, bacteriacausing slime formation mix from outer side and slimes formed in thecooling tower mix by exfoliating, so that sterilization for slimecontrol becomes necessary. Since, even if contaminants such as suspendedsolids and organic substances have been removed, bacteria contained inthe water to be treated proliferate on the surface of RO membrane todecrease flux of permeate, sterilization of water to be treated isperformed by adding a sterilizer to prevent fouling of membrane byproliferation of bacteria.

As a sterilizer for general water systems, a free chlorine agent such aschlorine and sodium hypochlorite is widely used which causes, as anoxidizing agent, deterioration of RO membrane and decline of efficiency.In order to ease oxidizing effect, Patent Document 1 (JP H01-104310A)proposes formation of chloramine by adding ammonium ion aftersterilization by a free chlorine agent. However, Patent Document 1 doesnot disclose application to water to be treated containing contaminantin detail, for instance, when and how to apply.

Patent document 2 (JP2006-263510A) teaches a slime control agent formembrane separation which comprises a combined chlorine agent comprisingan oxidizing agent based on chlorine and a sulfamic compound. Patentdocument 2 also teaches that free chlorine is contained in specificratio to the above combined chlorine agent added into water to betreated, and that there is a correlation similar to equilibrium. Inorder to obtain sterilizing effect, the above combined chlorine agent isto be used in a concentration wherein free chlorine is detected in thewater to be treated, while the combined chlorine agent is used in aconcentration wherein free chlorine concentration is 2-6 mg/L and totalchlorine concentration is 20-60 mg/L in the Examples.

Since there are problems that a RO membrane, especially a RO membranecomposed of high molecular weight membrane having a nitrogen-containinggroup such as polyamide and aramid is easily damaged by free chlorinedeteriorating membrane separation efficiency such as rejection rate andremoving rate, it is important to perform RO membrane treatment in acondition wherein free chlorine is not contained. For this, Patentdocument 3 (JP H09-57067A) proposes to carry out RO membrane treatmentafter sterilizing by free chlorine agent and then eliminating thesterilizing agent by adding reducing agent such as sodium bisulfite. InPatent document 3, copper concentration is restricted because effect ofelimination of sterilizing agent by addition of reducing agent isinsufficient. It is suggested that elimination of free chlorine aftersterilization by free chlorine agent is necessary.

Since, as above, a RO membrane, especially a RO membrane composed ofhigh molecular weight membrane having a nitrogen-containing group suchas polyamide and aramid is required to be used in a water systemcontaining no free chlorine, residual chlorine should be removed from afeed water sterilized with a chlorine agent before supplying to a ROmembrane. If, however, the feed water is supplied to a RO membrane afterresidual chlorine has been removed, slimes will be formed as thetreatment lasts to cause deterioration of membrane performance. In orderto avert these problems, a combined chlorine agent as shown in Patentdocument 2 can be added in a free chlorine concentration of 0.1 m/L orlower. This method may be possible in the case that the combinedchlorine agent is prepared on the spot to be added. However, in the casethat a combined chlorine agent prepared in a factory is added, afterstorage and transportation, on the spot so as to be diluted to a freechlorine concentration of 0.1 m/L or lower, there occurs a problem thatcombined chlorine (total chlorine) concentration becomes low to decreaseslime prevention effect. Accordingly, there is a demand for a combinedchlorine agent having a low concentration of free chlorine and a highconcentration of combined chlorine.

PRIOR TECHNICAL DOCUMENT Patent Document

Patent document 1: JP H01-104310A

Patent document 2: JP2006-263510A

Patent document 3: JP H09-57067A

SUMMARY OF INVENTION Subject to be Solved by Invention

An object of the present invention is to provide, in order to solve theabove prior problem, a combined chlorine agent having a lowconcentration of free chlorine and a high concentration of combinedchlorine whereby combined chlorine concentration of water can beheightened when added to water systems at a low concentration of freechlorine. Another object of the present invention is to provide a methodfor producing the combined chlorine agent efficiently. A further objectof the present invention is to provide a method of chlorine treatmentunder condition of a low free chlorine concentration.

Means to Solve Subject

The present invention consists in the following combined chlorine agent,method of production thereof and method of chlorine treatment;

(1) A combined chlorine agent consisting of an aqueous agent whichcomprises alkali consisting of alkali metal hydroxide, sulfamic acid,and oxidizing agent based on chlorine,

wherein the composition ratio of the oxidizing agent based on chlorineto the sulfamic acid is in the range from 0.45 to 0.6 by Cl/N (moleratio),

the composition ratio of the oxidizing agent based on chlorine to alkaliis in the range from 0.3 to 0.4 by Cl/alkali metal (mole ratio), and

free chlorine concentration in the aqueous agent is 2% by weight orlower of total chlorine concentration.

(2) The combined chlorine agent according to the above (1), wherein thecombined chlorine agent is used for slime control agent of a reverseosmosis membrane.

(3) A method of producing the combined chlorine agent according to theabove (1) or (2), comprising

adding sulfamic acid to an aqueous solution of alkali consisting ofalkali metal hydroxide to dissolve,

adding oxidizing agent based on chlorine to the obtained sulfamicacid—alkali aqueous mixture, and

mixing the finally obtained aqueous mixture to form an aqueous solutionagent.

(4) The method of producing the combined chlorine agent according to theabove (3), wherein the aqueous solution of alkali has a water content of50-65% by weight.

(5) A method of chlorine treatment, comprising

adding the combined chlorine agent according to the above (1) to a watersystem so that free chlorine concentration is 0.1 mg/L or lower.

(6) A method of chlorine treatment, comprising

adding the combined chlorine agent according to the above (1) to a feedwater system to a reverse osmosis membrane so that free chlorineconcentration is 0.1 mg/L or lower.

According to the present invention, free chlorine, combined chlorine,and total chlorine concentration are given in JIS K 0400-33-10:1999 andmeasured as Cl₂ concentration by DPD method usingN,N-diethyle-1,4-phenylenediamine. Free chlorine is ruled as chlorineresiding in a form of hypochlorous acid, hypochlorite ion or dissolvedchlorine. Combined chlorine is ruled as chlorine residing in a form ofchloramine or organic chloramine which is measured by the DPD method butexcluded from the free chlorine. Total chlorine is ruled as chlorine informs of free chlorine and/or combined chlorine.

A combined chlorine agent is an agent forming the above combinedchlorine. The combined chlorine agent according to the present inventionis an aqueous agent comprising alkali consisting of alkali metalhydroxide, sulfamic acid, and oxidizing agent based on chlorine. In thecombined chlorine agent of the present invention, the composition ratioof the oxidizing agent based on chlorine to the sulfamic acid in theaqueous agent is in the range from 0.45 to 0.6, preferably 0.45-0.55 byCl/N (mole ratio), the composition ratio of the oxidizing agent based onchlorine to alkali in the aqueous agent is in the range from 0.3 to 0.4,preferably 0.30-0.36 by Cl/alkali metal (mole ratio), and free chlorineconcentration in the aqueous agent is 2% by weight or lower of totalchlorine concentration. The aqueous solution agent is preferably of pH13 or more and the composition ratio of sulfamic acid to alkali in theaqueous solution agent is in the range from 0.5 to 0.7 by N/alkali metal(mole ratio). The above Cl/N (mole ratio) corresponds to the ratio ofCl₂ mole number of the oxidizing agent measured according to JIS K0400-33-10:1999 to mole number of sulfamic acid composed of N. The aboveN/alkali metal (mole ratio) corresponds to the ratio of the above molenumber of sulfamic acid to mole number of alkali metal formed by alkalimetal hydroxide.

Sulfamic acid to form the combined chlorine agent is amidosulfuric acidrepresented by

R¹R²NSO₃H  [1]

wherein R¹,R² denote each H or hydrocarbon group having carbon number of1-6 independently. Preferable sulfamic acid is the one in narrow sensein which R¹,R² denote each H, while N-methyl sulfamic acid, N,N-dimethylsulfamic acid, N-phenyl sulfamic acid, and so on may be used. Sulfamicacid may be used in the state of free acid (in powder) or sulfamate suchas alkali metal salt including sodium salt and potassium salt.

Alkali for composing a combined chlorine agent consists of alkali metalhydroxide such as sodium hydroxide and potassium hydroxide. An oxidizingagent based on chlorine includes hypochlorous acid, chlorous acid andwater soluble salt thereof such as alkali metal salt thereof. These arepreferably of no content of sodium chloride. By controlling sodiumchloride content in the aqueous agent at 50,000 mg/L or lower,precipitation of salt is prevented and stability of a halogenatedoxidizing agent becomes increased.

The combined chlorine agent is produced by a method wherein sulfamicacid is added to an aqueous solution of alkali consisting of alkalimetal hydroxide to dissolve, then an oxidizing agent based on chlorineis added to the obtained sulfamic acid—alkali aqueous mixture, and thefinally obtained aqueous mixture is mixed to form an aqueous solutionagent. The aqueous solution of alkali preferably contains 50-65% byweight of water. Alkali consists of alkali metal hydroxide such assodium hydroxide and potassium hydroxide which maintains solubility whenthe aqueous solution of the combined chlorine agent has formed.

Sulfamic acid may be added in a form of sulfamate. Usable sulfamateincludes sodium sulfamate, potassium sulfamate, and ammonium sulfamatewhich are soluble when the aqueous solution of the combined chlorineagent has formed. Sulfamic acid is added in order that sulfamic acidconcentration in the aqueous solution agent becomes to be the aboveconcentration. Amount of sulfamic acid to be added is preferably theamount wherein composition ratio of sulfamic acid to alkali is in therange from 0.5 to 0.7 by N/alkali metal (mole ratio). Sulfamic acid maybe added in a form of powder or aqueous solution of sulfamic acid orsulfamate. In the case using sulfamate, amount of alkali metal containedin sulfamate is calculated as alkali. In the case using an aqueoussolution, amount of water contained in the aqueous solution is added aswater in the aqueous solution of alkali.

Oxidizing agent based on chlorine is preferably hypochlorous acid orhypochlorite which is preferably added as an aqueous solution havingavailable chlorine (Cl₂) concentration of 5-20% by weight, preferably10-15% by weight. Additive amount of oxidizing agent based on chlorineis decided in order that combined chlorine concentration in the aqueoussolution agent becomes the above concentration as available chlorine(Cl₂) concentration, and that composition ratio of oxidizing agent basedon chlorine to sulfamic compound by Cl/N (mole ratio) becomes the aboveratio, whereby a combined chlorine agent of aqueous solution agentsuperior in reactivity, stability, handling and scentlessness ofchlorine is produced efficiently without foaming and emitting chlorineodor. Here, oxidizing agent based on chlorine is preferably added andmixed gradually.

The combined chlorine agent according to the present invention asproduced above is used by adding to water system for chlorine treatment.The agent has low free chlorine concentration and high combined chlorineconcentration, so that combined chlorine concentration of water systemto which the agent has been added can be heightened even when added at alow free chlorine concentration. Since chlorine in the combined chlorineagent is in a form decided by equilibrium between free chlorine andcombined chlorine (total chlorine), it is presumed that, even in case oflow free chlorine concentration, chlorine latently stored in a form ofcombined chlorine is released gradually to give an effect by a chlorineagent such as sterilization. Therefore, the water system added with thecombined chlorine agent is laid in a sterilization-active state so thatslime forming is prevented. The combined chlorine agent can be added towater system at a free chlorine concentration of 0.1 mg/L or lower toperform chlorine treatment. In this case, total chlorine concentrationmay be 1-50 mg/L.

The combined chlorine agent according to the present invention issuitably used for slime control agent for RO membrane. RO membrane is apermeation membrane to separate and remove solutes such as salts,organic matters and so on from water to be treated by reverse osmosis.RO membranes generally used for reverse osmosis membrane treatment areproper object of the present invention. For the material of the ROmembrane, the present invention is especially effective to a polymermembrane having nitrogen-containing group, such as polyamide (especiallyaromatic polyamide of low tolerance to chlorine), polyurea, andpolypiperazine amide, while other RO membranes based on such ascellulose acetate may be applicable. The RO membrane may have astructure of module such as spiral type, hollow fiber type, tubulartype, and flat membrane type.

The objective water to be treated by RO membrane treatment according tothe present invention may be water to be treated containingcontaminative substances. Such water to be treated is subjected topretreatment for removing contaminative substances under presence offree chlorine to cancel contaminative power to RO membrane and tocontrol free chlorine concentration to be 0.1 mg/L or lower, whereby ROmembrane treatment can be performed efficiently while preventingcontamination and deterioration of RO membrane. In order to control thefree chlorine concentration of pretreated water being the above value, areducing agent may be added to the pretreated water. According to thepresent invention, such water to be treated for RO membrane is subjectedto chlorine treatment by adding the combined chlorine agent to controlthe free chlorine concentration to be 0.1 mg/L or lower, whereby slimecontrol of RO membrane can be performed. Here, total chlorineconcentration may be 1-50 mg/L.

Effect of Invention

According to the present invention, a combined chlorine agent having alow concentration of free chlorine and a high concentration of combinedchlorine can be obtained, whereby combined chlorine concentration can beheightened even if added at a low free chlorine concentration.

According to the method for producing the combined chlorine agent of thepresent invention, such combined chlorine agent can be producedefficiently.

According to the method of chlorine treatment of the present invention,chlorine treatment under condition of a low free chlorine concentrationcan be performed by adding the above combined chlorine agent. In thecase that the combined chlorine agent is used for slime control agent ofRO membrane, slime control is performed without damage of RO membranewhereby RO membrane treatment can be carried out efficiently.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing correlation between Cl/N (mol ratio) and freechlorine concentration in Examples 1-3 and Comparative Examples 1-3.

FIG. 2 is a graph showing variations of permeate and pressure drop inExample 4.

FIG. 3 is a graph showing variation of salt rejection in Example 4.

FIG. 4 is a graph showing variation of permeate and pressure drop inComparative Example 9.

FIG. 5 is a graph showing variation of salt rejection in ComparativeExample 9.

FIG. 6 is a graph showing variation of permeate and pressure drop inComparative Example 10.

FIG. 7 is a graph showing variation of salt rejection in ComparativeExample 10.

EMBODIMENT OF INVENTION

Below, the present invention is explained by way of Examples andComparative Examples wherein % denotes “% by weight” and part denotes“part by weight” unless specific indication is given.

EXAMPLES Examples 1-3

To pure water in amount shown for each Example in Table 1, sodiumhydroxide was added to dissolve, then sulfamic acid (powdery sulfamicacid wherein R¹,R² in the above formula 1 denote each H) was added todissolve, and then sodium hypochlorite in amount shown for each Examplein Table 1 was added to dissolve whereby combined chlorine agent ofaqueous solution agent of each Example was produced. Characteristics,free chlorine concentration and total chlorine concentration of theobtained aqueous solution agents ware shown in Table 1. In Tables 1-4,NaOH (Na mol/L), N (mol/L) and available chlorine (mol/L) ware shown asvalue each calculated as specific gravity of the solution is 1.3.

TABLE 1 Example 1 Example 2 Example 3 Main component Comb. Cl Comb. ClComb. Cl agent agent agent Pure water (part) 15.5 13.2 8.6 48% NaOH(part) 19.5 20.8 23.4 Sulfamic acid (part) 15.0 16.0 18.0 12% (Cl₂)NaClO (part) 50.0 50.0 50.0 Total (part) 100 100 100 NaOH (Na mol/L)3.043 3.246 3.651 N (mol/L) 2.009 2.143 2.411 Available Cl (mol/L) 1.0991.099 1.099 N/Na (mol ratio) 0.66 0.66 0.66 Cl/N (mol ratio) 0.55 0.510.46 Cl/Na (mol ratio) 0.36 0.34 0.30 pH (when prepared) pH > 13 pH > 13pH > 13 Free Cl (when prepared) (mg/L) <1000 <1000 <1000 Total Cl (whenprepared) (%) 6.9 6.9 6.9 Appearance (when prepared) Good Good Good pH(after 7 days storage at 40° C.) pH > 13 pH > 13 pH > 13 Free Cl (after7 days storage 930 920 940 at 40° C.) (mg/L) Total Cl (after 7 daysstorage 6.9 6.9 6.9 at 40° C.) (%) Appearance Good Good Good (after 7days storage at 40° C.) Appearance Good Good Good (after 7 days storageat −5° C.)

Comparative Examples 1-3

Examples 1-3 were pursued except that composition of each component waschanged as described in Table 2. Results of Comparative Examples 1-3ware shown in Table 2. In Comparative Example 1, two parts ofbenzotriazole ware added as another component to sum total amount of 100parts which was shown with *1 in Table 2.

TABLE 2 Comp. Comp. Comp. Example 1 Example 2 Example 3 Main componentComb. Cl Comb. Cl Comb. Cl agent agent agent Pure water (part) 6.7 12.46.6 48% NaOH (part) 19.3 15.6 18.9 Sulfamic acid (part) 12.0 12.0 14.512% (Cl₂) NaClO (part) 60.0 60.0 60.0 Total (part) 100 *1 100 100 NaOH(Na mol/L) 3.012 2.434 2.949 N (mol/L) 1.607 1.607 1.942 AvailableCl(mol/L) 1.319 1.319 1.319 N/Na (mol ratio) 0.53 0.66 0.66 Cl/N (molratio) 0.82 0.82 0.68 Cl/Na (mol ratio) 0.44 0.54 0.45 pH (whenprepared) pH > 13 pH > 13 pH > 13 Free Cl (when prepared) (mg/L) 60006000 2000 Total Cl (when prepared) (%) 7.7 7.7 7.7 Appearance (whenprepared) Good Good Good pH (after 7 days storage at 40° C.) pH > 13pH > 13 pH > 13 Free Cl (after 7 days storage 8000 — — at 40° C.) (mg/L)Total Cl (after 7 days storage 7.1 — — at 40° C.) (%) Appearance GoodGood Good (after 7 days storage at 40° C.) Appearance Good Good Good(after 7 days storage at −5° C.)

Comparative Examples 4-6

Examples 1-3 were pursued except that composition of each component waschanged as described in Table 3, wherein sulfamic acid was not able todissolve into the aqueous solution of pure water added with sodiumhydroxide (shown as “Deposit”). Results of Comparative Examples 4-6 wareshown in Table 3.

TABLE 3 Comp. Comp. Comp. Example 4 Example 5 Example 6 Main componentComb. Cl Comb. Cl Comb. Cl agent agent agent Pure water (part) 6.6 4.06.7 48% NaOH (part) 24.4 26.0 27.3 Sulfamic acid (part) 19.0 20.0 21.012% (Cl₂) NaClO (part) 50.0 50.0 45.0 Total (part) 100 100 100 NaOH (Namol/L) 3.808 4.057 4.260 N (mol/L) 2.545 2.678 2.812 Available Cl(mol/L) 1.099 1.099 0.989 N/Na (mol ratio) 0.67 0.66 0.66 Cl/N (molratio) 0.43 0.41 0.35 Cl/Na (mol ratio) 0.29 0.27 0.23 pH (whenprepared) — — — Free Cl (when prepared) (mg/L) — — — Total Cl (whenprepared) (%) — — — Appearance (when prepared) Deposit Deposit DepositpH (after 7 days storage at 40° C.) — — — Free Cl(after 7 days storage —— — at 40° C.) (mg/L) Total Cl (after 7 days storage — — — at 40° C.)(%) Appearance — — — (after 7 days storage at 40° C.) Appearance — — —(after 7 days storage at −5° C.)

Comparative Examples 7-8

Examples 1-3 were pursued except that composition of each component waschanged as described in Table 4, wherein sulfamic acid was not able todissolve into the aqueous solution of pure water added with sodiumhydroxide. Results of Comparative Examples 7-8 ware shown in Table 4.

TABLE 4 Comp. Comp. Example 7 Example 8 Main component Comb. Comb. Clagent Cl agent Pure water (part) 13.6 9.0 48% NaOH (part) 23.4 26.0Sulfamic acid (part) 18.0 20.0 12% (Cl₂) NaClO (part) 45.0 45.0 Total(part) 100 100 NaOH (Na mol/L) 3.651 4.057 N (mol/L) 2.411 2.678Available Cl (mol/L) 0.989 0.989 N/Na (mol ratio) 0.66 0.66 Cl/N (molratio) 0.41 0.37 Cl/Na (mol ratio) 0.27 0.24 pH (when prepared) — — FreeCl (when prepared) (mg/L) — — Total Cl (when prepared) (%) — —Appearance (when prepared) Deposit Deposit pH(after 7 days storage at40° C.) — — Free Cl(after 7 days storage — — at 40° C.) (mg/L) Total Cl(after 7 days storage — — at 40° C.) (%) Appearance (after 7 daysstorage — — at 40° C.) Appearance (after 7 days storage — — at −5° C.)

In Examples 1-3 and Comparative Examples 1-3, correlation between Freechlorine concentration/Sulfamic acid (mol ratio), namely Cl/N (molratio) and Free chlorine concentration is shown in FIG. 1, wherein freechlorine concentrations in Comparative Examples 1-3 ware calculated sothat the total chlorine concentration ware 6.9% as same as Examples 1-3.

The above result shows that, in Examples 1-3, free chlorineconcentration in the aqueous agent is 1000 mg/L or lower which falls in2% by weight or lower of total chlorine concentration and that inComparative Examples 1-3, free chlorine concentration in the aqueousagent is higher than 1000 mg/L which is higher than 2% by weight oftotal chlorine concentration. Although total chlorine concentration inExamples 1-3 is lower than in Comparative Examples 1-3, the differenceof total chlorine concentration is smaller than the difference of freechlorine concentration. It is also shown that, in Comparative Examples4-8, an aqueous agent can not be produced because deposit occurs.

Examples 4

Cooling water discharged from a cooling tower was added with a combinedchlorine agent and subjected to coagulation treatment, filtration andactive carbon treatment to obtain a pretreated water having totalchlorine concentration of 5 mg/L and free chlorine concentration of 0.5mg/L. The pretreated water was added with 10% by weight of aqueoussolution of sodium bisulfite was added at sodium bisulfite concentrationof 15 mg/L to reduce total chlorine and free chlorine contained in thepretreated water_to zero whereby water to be treated was prepared. Tothis water to be treated, combined chlorine agent obtained in Example 2was added so that total chlorine concentration was 1.2 mg/L and freechlorine concentration 0.05 mg/L. The water to be treated waspressurized to 1.5 MPa by a pump and supplied to a concentrate room of aRO membrane treatment apparatus to carry out RO treatment. The ROmembrane treatment apparatus was installed with one of 4 inch spiraltype RO membrane element of aromatic polyamide (“ES 20-D4” of NittoElectric Industrial Co., Ltd) in a vessel.

During 3 month continuous operation under the above condition, nodeterioration of RO membrane, no increase of pressure drop and no slimetrouble occurred. In the period, variations of permeate and pressuredrop are shown in FIG. 2 and variation of salt rejection is shown inFIG. 3 wherein it is affirmed that there occurs no clogging by slimebecause there is no increase of pressure drop and no decrease of fluxsince the start of the operation. Further, it is also affirmed thatthere occurs no deteriorate of water quality of produced water byadhesion of contaminant to RO membrane or deterioration of RO membranebecause there occurs no decrease of salt rejection since the start ofthe operation.

Comparative Examples 9

Examples 4 was pursued except that pretreated water to be supplied to ROmembrane was further added with a chlorine agent so that total chlorineconcentration was 13 mg/L and free chlorine concentration 0.2 mg/L. Inthe period, variation of permeate and pressure drop is shown in FIG. 4and variation of salt rejection is shown in FIG. 5 wherein it isrecognized that there occurs deterioration of RO membrane.

Comparative Examples 10

Examples 4 was pursued except that the water to be treated in thepretreatment was not added with a chlorine agent, and that pretreatedwater of free chlorine concentration of 0.0 mg/L was supplied to ROmembrane. In the period, variations of permeate and pressure drop areshown in FIG. 6 and variation of salt rejection is shown in FIG. 7wherein it is recognized that there occurs decrease of flux by slimeformation.

INDUSTRIAL APPLICATION OF THE INVENTION

The present invention can be applied to a combined chlorine agent usedfor a slime control agent of reverse osmosis membrane and used for otherchlorine treatment agents, method of production thereof and method ofchlorine treatment as the use thereof, especially method of chlorinetreatment for slime control of reverse osmosis membrane.

1. A combined chlorine agent consisting of an aqueous agent whichcomprises alkali consisting of alkali metal hydroxide, sulfamic acid,and oxidizing agent based on chlorine, wherein the composition ratio ofthe oxidizing agent based on chlorine to the sulfamic acid is in therange from 0.45 to 0.6 by Cl/N (mole ratio), the composition ratio ofthe oxidizing agent based on chlorine to alkali is in the range from 0.3to 0.4 by Cl/alkali metal (mole ratio), and free chlorine concentrationin the aqueous agent is 2% by weight or lower of total chlorineconcentration.
 2. The combined chlorine agent as claimed in claim 1,wherein the combined chlorine agent is used for slime control agent of areverse osmosis membrane.
 3. A method of producing the combined chlorineagent as claimed in claim 1, comprising adding sulfamic acid to anaqueous solution of alkali consisting of alkali metal hydroxide todissolve, adding oxidizing agent based on chlorine to the obtainedsulfamic acid-alkali aqueous mixture, and mixing the finally obtainedaqueous mixture to form an aqueous solution agent.
 4. The method ofproducing the combined chlorine agent as claimed in claim 3, wherein theaqueous solution of alkali has a water content of 50-65% by weight.
 5. Amethod of chlorine treatment, comprising adding the combined chlorineagent as claimed in claim 1 to a water system so that free chlorineconcentration is 0.1 mg/L or lower.
 6. A method of chlorine treatment,comprising adding the combined chlorine agent as claimed in claim 1 to afeed water system to a reverse osmosis membrane so that free chlorineconcentration is 0.1 mg/L or lower.